The present invention relates to a method of making a ferrite material powder by an spray pyrolysis process and a method of producing a ferrite magnet using the material powder.
Ferrite is a generic term for various compounds containing an oxide of a divalent cationic metal and an oxide of trivalent iron. Ferrite magnets have found a wide variety of applications in motors, loudspeakers, and so on. Typical materials for ferrite magnets include Sr or Ba ferrites with an M-type magnetoplumbite hexagonal structure (SrFe12O19 or BaFe12O19).
A basic composition of the M-type magnetoplumbite ferrite is usually represented by the chemical formula of AO.6Fe2O3, where A is a metal element that exhibits divalent cation and is selected from the group consisting of Sr, Ba, Pb, Ca and so on. Each of these ferrites can be made from iron oxide and a carbonate of strontium (Sr) or barium (Ba) at a relatively low cost by a powder metallurgical process.
The conventional process, however, has the following drawbacks:
1) Material powders are usually mixed and dispersed in their solid phase, and cannot always be mixed uniformly. That is to say, the composition of the resultant mixture is not uniform enough. Because the magnetic properties of the material are proportional to the uniformity of the mix, the magnetic properties of a product made from the mixture like this cannot always reach sufficiently high levels.
2) The material powder is normally calcined at as high a temperature as 1150 to 1400xc2x0 C., thus raising the production costs.
3) The material powder often increases its particle size during the calcination process. Accordingly, it takes a long time to grind the powder mechanically down to a size of 1 xcexcm or less using a ball mill, for example, in the next pulverization process. In addition, since the grinding medium is worn to a certain degree every time this pulverization process is carried out, some impurities, i.e., particles of grinding medium, might be mixed into the powder, and the composition of the powder might deviate as a result. Furthermore, even after the material powder has been pulverized finely, the powder still does not have a sufficiently sharp particle size distribution. Thus, the magnetic properties of a final product are likely to deteriorate due to all of these problems.
It is therefore an object of the present invention to provide a ferrite material powder of fine and highly uniform particle sizes and with a uniform composition.
An inventive method of making a magnetoplumbite-type(M-type) ferrite material powder includes the step of preparing the ferrite material powder by spraying a mixed chloride solution, in which a chloride of iron and a chloride of strontium are dissolved, into a heated atmosphere. The solution of the mixed chloride contains approximately 25% to 35% of the chloride of iron and 2.4% to 4.9% of the chloride of strontium.
Another inventive method of making a M-type ferrite material powder includes the step of preparing the ferrite material powder by spraying a mixed chloride solution, in which a chloride of iron and a chloride of strontium are dissolved, into a heated atmosphere. The mixed chloride solution further contains: at least one chloride selected from the group consisting of a chloride of cobalt, a chloride of manganese and a chloride of nickel; and a chloride of lanthanum.
In one embodiment of the present invention, the mixed chloride solution preferably contains: 25% to 35% of the chloride of iron; 1.9% to 4.9% of the chloride of strontium; 0.09% to 1.0% of the at least one chloride selected from the group consisting of the chloride of cobalt, the combined chloride of manganese and chloride of nickel; and 0.16% to 2.0% of the chloride of lanthanum.
In another embodiment of the present invention, a combustible solvent may be mixed into the mixed chloride solution.
Still another inventive method of making an M-type ferrite material powder includes the steps of: spraying a mixed chloride solution, in which a chloride of iron and a chloride of strontium are dissolved, into a heated atmosphere inside a roasting furnace, thereby pyrolizing the mixed chloride solution into the M-type ferrite material powder; and blowing a combustible gas and/or oxygen gas toward a zone where the mixed chloride solution is pyrolyzed to the M-type ferrite material powder.
Yet another inventive method of making an M-type ferrite material powder includes the steps of: spraying a mixed chloride solution, in which a chloride of iron and a chloride of strontium are dissolved, into a heated atmosphere inside a roasting furnace, thereby pyrolyzing the mixed chloride solution to the M-type ferrite material powder; and spraying a combustible solvent toward a zone where the mixed chloride solution is pyrolyzed to the M-type ferrite material powder.
In one embodiment of the present invention, a chloride of lanthanum, and at least one chloride selected from the group consisting of a chloride of cobalt, a chloride of manganese and a chloride of nickel are dissolved in the mixed chloride solution, and the mixed chloride solution is acidic.
In another embodiment of the present invention, the mixed chloride solution is sprayed into the furnace preferably at a temperature between 800xc2x0 C. and 1300xc2x0 C., more preferably at a temperature between 900xc2x0 C. and 1200xc2x0 C.
In still another embodiment, the concentration of the chloride of iron in the mixed chloride solution is preferably from 27% to 33%.
In yet another embodiment, a waste produced by acid cleaning at an ironworks may be used as a material for the mixed chloride solution. In this particular embodiment, the mixed chloride solution may be sprayed using a hydrochloric acid collector at the ironworks.
In yet another embodiment, the inventive method may further include the step of conducting a heat treatment on the ferrite material powder. In that case, the heat treatment is conducted preferably at a temperature between 800xc2x0 C. and 1200xc2x0 C., more preferably at a temperature between 900xc2x0 C. and 1050xc2x0 C.
An inventive method of producing a ferrite magnet includes the steps of: preparing the M-type ferrite material powder that has been made by the inventive method of making the M-type ferrite material powder; and producing a permanent magnet from the ferrite material powder.